There have been many threads here and elsewhere about the real color of coroguard and how to paint it onto your model. Coroguard is a carrier into which powdered aluminum is added and then applied to the wing surface. It goes on a bit rough, which is why its appearance varies under different lighting conditions. Below are pictures of the same A330 wing under different lighting conditions, all in the same day.

Basically, this gives a modeler great latitude in representing the color. I display my models inside with scattered natural light that most closely resembles the real aircraft under overcast, so I use a medium gray mixed with an aluminum color to get that appearance: